This invention relates to a method and apparatus for cleaning the surface of objects using a blast of particles. Particle blast cleaning is well known in the art. The most common form of particle blast cleaning is sandblasting in which fine abrasive particles of sand or other grit are propelled by a jet of high pressure air against the surface of the object to be cleaned. Another form of particle blast cleaning uses small particles or pellets of solidified carbon dioxide as the abrasive which is propelled by a jet of high pressure air or nitrogen against the surface of the object to be cleaned. Carbon dioxide particles or pellets have the advantage over sand or grit in that the carbon dioxide particles or pellets will not work harden the surface of the object being cleaned and will sublimate after they strike the surface to be cleaned. Therefore, solid carbon dioxide particles and pellets are environmentally cleaner and safer to use and save the labor and expense usually needed to clean up the spent sand or grit used in sandblasting.
The solid carbon dioxide abrasives used for blast cleaning are usually small solid carbon dioxide pellets produced by compressing solid phase carbon dioxide snow in a pelletizer or compressing and extruding solid phase carbon dioxide snow in an extruder, such as described in U.S. Pat. No. 4,727,687 to Moore. The known carbon dioxide blast cleaning equipment has usually used compressed air as the propellent to project the pellets against the surface to be cleaned because of the low cost and availability of compressed air in many fabrication shops and locations. In some cases nitrogen has also been used as the propellent.
Blast cleaning systems that use compressed air as the propellent require large compressors, dryers and filters to achieve sufficient compressed air pressures and volumes and to remove moisture and contaminants, such as oil, from the compressed air. The compressors, dryers and filters must be sized to supply sufficient air for maximum cleaning operations which usually dictates that very large compressors, dryers and filters must be used. If the cleaning is to be performed in the field rather than a shop, the compressors, dryers, filters and other auxiliary equipment must be mounted on a mobile carrier, such as a tractor-trailer, with a driver and compressor operator required to operate and maintain the air compressor facilities.
For some field blast cleaning operations, high pressure nitrogen contained in steel cylinders has been used as the propellent to avoid the necessity for the large and expensive air compressors and dryers, but this approach has been not only quite expensive because of the cost of nitrogen but also requires a plurality of large nitrogen cylinder trailers at the cleaning site to provide a continuous and sufficient supply of propellent. The use of compressed air also introduced the possibility of contaminants, such as oil or other foreign materials, to enter the system and end up contaminating the surface being cleaned and the blast cleaning equipment itself.